Responses in spikelet fertility and grain weight to high temperature (HT) at anthesis and the relation to floral positions within a panicle were investigated using two HT-susceptible cultivars (Xieqingzao and Gang46) and two HT-tolerant cultivars (Qianjiang3 and Haunghuazan) under well-controlled climatic conditions. Results showed that effects of HT at anthesis were more adverse for spikelet fertility than for grain weight. The cultivar-dependent difference in spikelet fertility response to HT was mainly attributed to the extent of decrease in pollen viability for HT exposure. HT at anthesis impelled the flowering date of the florets on the middle and lower parts within a panicle, thereby resulting in shortened duration to floret-opening date and relatively concentrated floret-blossoming date. Change in spikelet fertility induced by HT was more notable than that caused by different grain positions, and the effect of HT on pollen viability was independent of the floret positions on the rachides within a panicle. Positional differences in spikelet fertility and grain weight under the HT regime were closely associated with the duration of HT exposure, in addition to differences in competing ability for supply of assimilates after successful fertilisation.